Such a sensor, which is configured as a gas sensor, is known, for example, from U.S. Pat. No. 5,081,998 A. An IR radiation source is provided therein, which acts upon a total of four detectors by way of a filter arrangement. The filter arrangement has two filters having different pass characteristics. A first filter has a pass band for IR radiation that is absorbed by CO2. That filter is therefore also referred to as a “CO2 filter”. The detectors arranged downstream are designated CO2 detectors. The other filter has a pass band different therefrom which serves for determining a reference quantity. The detectors arranged downstream of that reference filter are referred to as reference detectors. Between the IR source and the two filters there is arranged a third filter which is referred to as a natural density filter and overlaps half of the first filter and half of the second filter. Accordingly, one of the two CO2 detectors and one of the reference detectors receives only IR radiation that has passed both through the natural density filter and through either the CO2 filter or the reference filter. In the evaluating device, the difference of the output signals of the two CO2 detectors and the difference of the two reference detectors is formed. The two differences are then divided by one another. Such a CO2 sensor is required, for example, for determining CO2 in a patient's breath so as to be better able to monitor the patient during anaesthesia.
A disadvantage of such sensors is that they have a relatively high power requirement, and another disadvantage is the number of detectors required. The arrangement known from U.S. Pat. No. 5,081,998 A requires a source of radiation which, in any case for prolonged use, makes it unsuitable for battery-operated use. Furthermore, such an IR source generally requires a certain heating-up period, so that without a degree of prior preparation it is not always possible to carry out measurements when desired.
The problem underlying the invention is to simplify the use of an IR sensor, which is introduced in the sensor described in US 2008/0283753, wherein the pass band of a first filter is arranged within the pass band of a second filter and the evaluating device forms the difference of the signals of the detectors and normalizes it to the signal of a detector.
That configuration makes it possible to evaluate substantially more IR radiation. The IR radiation is therefore not divided into two separate ranges, with each detector detecting only one range. Instead, one detector detects IR radiation having a pre-set spectral range, which also includes, for example, the absorption spectrum of the gas being determined, here CO2. The other detector detects an IR spectrum from a sub-range thereof, which does not include the absorption spectrum of the gas being determined. The normalization of the difference to the output signal of a detector enables fluctuations in the intensity of the IR radiation to be compensated. It is also possible to use more than two sensors with a correspondingly greater number of filters, the individual pass ranges then overlapping accordingly. With such a sensor it is also possible to obtain other information, for example relating to temperature, to movement in the room, to the number of persons in the room, etc. Because it is possible to detect substantially more radiation, the power consumption can be reduced, so that the necessary power can also be supplied by a battery. That in turn gives greater freedom in terms of local mounting and use. The sensor can transmit its signals wirelessly.
The pass band of the first filter is preferably larger than the pass band of the second filter. Accordingly, the first filter, in addition to including the spectral range allowed to pass by the second filter, also includes the spectral range in which IR radiation is absorbed.
The two filters preferably have a common cut-off wavelength. That simplifies evaluation. The difference between the output signals of the detectors can then readily be formed without additional calculation steps being necessary. The cut-off wavelengths are the wavelengths that define, that is to say limit, the pass bands. They are referred to as “lower wavelength” and “upper wavelength”.
It is how ever a known situation, that the amount as well as the spectral distribution of radiation of an emitter has a dependence of the temperature of the emitter. This is given by the well known Planck's distribution of radiation. Given a temperature of the emitter, a Planck curve then gives the dependence of the radiation to the wavelength, where the Planck curves has a maximum radiation at some wavelength, the maximum radiation value as well as the wavelength of the maximum radiation being temperature dependent.
Using a natural source in sensor systems such like the one described in for example US 2008/0283753, would make the pass bands of the filters change in energy (or in other words, the radiation intensity density) over the band of wavelengths.
This construction is able to compensate for changes in the intensity of radiation of the light source, however, is not robust to for example temperature changes of the light source.
It is one object of the present invention to introduce methods to solve these problems of the present sensors, and a sensor utilizing the solutions.